It goes without saying that packages intended to contain liquids and a variety of other materials should be capable of establishing a seal that is effective to prevent leakage. A packaging system which is becoming quite popular for retaining a wide variety of such materials employs an injection blow molded container, which either can be opaque, translucent or transparent, and an injection molded closure therefor. Injection blow molded containers or bottles can be made from a variety of different plastics, with a preferred plastic being a polyvinyl chloride that is formulated for use in an injection blow molding process, as is well known in the art. A threaded closure for such a bottle or container can be made of polypropylene or other similar plastic material.
One way of providing a seal for a threaded container is to employ a threaded cap with an internal liner therein. However, this introduces an additional element into the construction, and thereby introduces additional material and labor costs into the fabricating process. Moreover, in the event that the liner becomes deteriorated or otherwise worn, the sealing capabilities thereof can be adversely effected, thereby resulting in undesired leakage. Leakage also can occur when the internal pressure in the container causes the top to "dome", thereby adversely affecting the seal between the liner and the upper surface of the container. When the material being packaged is either caustic or otherwise harmful, the leakage of such a material is of a significant concern.
In order to overcome the problems associated with introducing an additional liner into the closure construction, it has been suggested to provide a threaded closure having an internal, cylindrical rib or valve for frictionally engaging the inner surface of a bottle, adjacent the upper surface of the finish thereof. However, it has been determined that the high forces needed to establish a leak-proof seal impose such high stresses in the upper region of the bottle finish that the finish of the bottle often cracks or splits. This problem has been found to be particularly acute with injection blow molded bottles made from polyvinyl chloride.